Method of fabrication of metals by pressure casting

ABSTRACT

A vehicle wheel is made of cast steel and has a thickness of steel small with respect to its diameter. The steel has a carbon content within the range of 0.16 to 0.70 percent, a manganese content within the range of 0 to 1.60 percent, and a silicon content within the range of 0 to 1.00 percent. The tensile strength of the steel is within the range of 40 kg/mm2 to 80 kg/mm2, and the steel is hardened through about 10 to 20 percent of its thickness. The thickness of the steel is increased in the rim flange as compared to the rim base, and the axially outer wall of the rim flange is inclined with respect to the rim axis. Any shrink marks that may appear in the cast steel are concentrated in the thickened portion of the rim flange.

Inventor Daniel Lejeune Clermont-Ferrand, France Appl. No. 648,397 Filed June 23, 1967 Patented Sept. 29, 1970 Assignee Compagnie Generale des Etahlissements Michelin, raison sociale Michelin & Cie, Clermont-Ferrand, Puy-de-Dome, France Priority Julie 24, 1966 France 66,940

METHOD OF FABRICATION 0F METALS BY PRESSURE CASTING 4 Claims, 5 Drawing Figs.

US. Cl 164/120,

Int. Cl 822d 27/14 Field 01' Search 164/1 19,

References Cited UNlTED STATES PATENTS 3,302,919 2/1967 Beetle et a1. l64/348X 779,192 l/1905 Vellino l64/348X 1,912,981 6/1933 Hoy 164/119X 2,990,592 7/1961 l'lursen 164/155 3,318,370 5/1967 Groteke et al. 164/306 Primary Examiner-J. Spencer Overholser Assistant Examiner-R. Spencer Annear I A ttorney- Brumbaugh, Graves, Donohue and Raymond ABSTRACT: The disclosure herein relates to high speed precision casting of metal articles wherein a carriage carrying a mold is positioned above a feed pipe entering an enclosure provided with ladle filled with liquid metal. The enclosure is then put under pressure to expel the liquid metal from the ladle into the mold through the feed pipe. The metal fills the cavity of the mold expelling the air contained therein through vents until the metal solidifies in the vents and closes them. The pneumatic pressure of the enclosure is increased during the progressive solidification of the metalfrom the' top to the bottom of the mold until the solidification front reaches the head of the feed pipe at which time the pressure is decreased, the core withdrawn and'the article separated from the mold.

Patented Sept. 29, 1970 Sheet INVENTOR. DANIEL L'EJEUNE Y v l 13W, @flM- v pg, :tATTORNEYS I ,Patented Sept. 29, 1970 She et l l l I I I I I INVENTOR. DANIEL LEJEUNE BY a 214% i his; I ,Arramvsys Patented Sept. 29, 1970 Sheet his ATTORNEYS Patented Sept. 29, 1970 Sheet INVENTOR. .DANIEL LEJEUNE his ATTORNEYS METHOD OF FABRICATION OF METALS BYPRESSURE CASTING The presentinvention relates to the manufacture of various metal products with outstanding economy by injection molding and, more particularly, to methods of and apparatus for high-speed, precision casting of metal articles of many configurations, including portions shaped as surfaces of revolution.

Conventional metal founding offers the advantage of furnishing formed pieces regardless of their complexity directly from the liquid metal. However, this advantage is offset by the many disadvantages whichinfluence the production costs. Specifically, conventional metal founding requires the handling of weights which'are very large in comparison to the weight of the finished-product, including heavy'sand'and'tools, such as the moldbox'es which must be used, as well as the bandling of the metal'itself either inliquid condition in containers which are also heavy and voluminous or insolid condition in the form of crude pieces which have to be subjected to a plurality of finishing operations;

Secondly, conventional metal founding involves slow and very sensitive thermic phenomena requiring varying: conditions of handling and control. This makes it impossible to automate the entire production cycle of an article which otherwise could be manufactured on a large scale and at low cost if automation were possible.

Thirdly, the conditions present during metal feeding and tapping usually cause the weight of the metal put into production to be more than double the weight of the piece itself.

Finally, tolerances of finished pieces are necessarily broad, due to the opposing requirements to which the materials out of which'the molds and cores are manufactured are subjected.

' if pieces with fine tolerances are desired, this necessitates burdensome machining after casting itself has been completed.

In accordance with the present invention, the method of casting metal pieces consists of the following steps:

I First: Filling a bottom-filling mold, with liquid metal being delivered by pneumatic pressure from a casting ladle and entering the mold through a feed pipe connected with the bottom of the mold; 7

Second: Closing the gas escape vents contained in the [hold by solidifying the metal which entered these vents, this operation'being facilitated by the small opening of each vent, all this being done in order to put the liquid metal in the mold under pressure;

Tiiirdi Continuing or increasing the pneumatic pressuretransmitted through the liquid metal until it cools to the liquidsolid interface; I V g g V H W F mirth: Causing the cooling front, i.e., the liquid-solid inter face, to progress toward the filling opening in the bottom of themold to maintain the entire liquid phase in communication with the bottom either by artificial cooling of the parts of the mold or the core which are in contact with the massive por tions of the piece, or by varying the cross section of the mold or of the core thereby accelerating the cooling of the massive portions and/or slowing the cooling of the thin portions;

Fifihflieducing pneumatic pressure abruptly when the cooling front reaches the bottom of the piece, and causing the separation of the piece from the molten casting metal to enable the liquid metal to flow back into the casting ladle.

In accordance with the present invention, the entire molding cycle is carried out at a speed such that the difference of temperatures between the first point of solidification and the last point of solidification does not exceed approximately 50C when pneumatic pressure is reduced so that at that time, the piece in its entirety is in pasty condition and it becomes possible to extract from the mold the workpiece or the withdrawable core giving shape to the inside of the piece prior to the contraction of the workpie ce. l 'or example, apiece weighing l kg and having an average thickness of about 6 mmis cast within 8 to 10 seconds and a piece weighing 40 kg having an average thickness of 10 mm is cast within 10 to seconds.

The method in accordance with the present invention is carried out in the following manner.

The first stage is that of fillingof the mold. The metal is conveyed by pneumatic pressure from the, casting ladle to the mold to facilitate automatization of feeding, at controlled speed and at large deliveries, without stirringor-pollutionof the. liquid bath.

The second: and third stages are those of puttingthe liquid metal in the. mold under pressure: the mold being full, the metal following in the path of the gases which were; contained in the mold enters the vents, and due to the vents small cross section, closes them by solidifying, the metal therein instantaneously. Thesevents may be quite simply aslightclearance in the joints. of the mold. In the third stage, pressure is increased to offset the shrinkage on solidification and prevent shrinkage holes and internal defects which may be caused by an insufficiency of metal.

The fourth stage isconcerned withthe coolingprocess. Regardless of the shape andthe cross section of the piece to be cast, it is advisable to avoid bottling up a liquid pocket between solidified walls which isolates the pocket; from the feeding liquid metal under pressure and results in an insufficiency of metal and thus shrinkage. holes on solidification. Accelerationor slowing down of cooling,,depending on the case, makes it possible to prevent the. formation of shrinkage holes; In general, the cooling rate can be controlled by providing mold or core walls of suitable thickness. In exceptional cases, cooling by circulation of a cooling fluid may be necessary. The form of the piece to be manufactured determines the zones forwhich acceleration or retarding of cooling will be required.

The fifth stage is the separation of the piece. In order to assure such separation, in spite of the possible presence of a film of solidified metal on the inner walls. of the feedv pipe, a small collar, located at the joint of the bottom of the mold and the head of the feed pipe, is reheated or its cooling rate retarded to ensure the presence of a zone in which the metal remains liquid in the breaking-off zone. In addition, one may also provide for moving the mold with respect to the pipe in order tofacilitate separation.

In accordance with a preferred arrangement of the invention, graphite molds are used. The use of graphite molds offers many advantages. Graphites expansion coefficient, and thusits shrinkage on cooling, is very lo wfThe low shrinkage of graphite molds in turn permits precision in the production of t sz ss r s syl ad it s qt ya nq t li teyhess er of the gases enclosed in the liquid metal, as do the best molding sands, without itself giving off any gas. Consequently, there are no blisters on the cast pieces. Graphite may be machined to an excellent polish and this gives the cast piecesan ite facilitates extra-rapid cooling and this on the one hand,

makes it possible to separate the surface of the piece due to the formation of a skin which, by contraction, detaches itself from the mold and, on the other hand accelerates the thermic phenomeon and facilitates automatization of the casting The high heat conductivity of graphite also permits direct contact with liquid steel, the cooling of the steel being more rapid as the thickness of the graphite increases. To protect the mold from deterioration by the high temperatures, the graphite may be protected'by black lead or'oth'er luting matfial. Lastly, graphite may be used Without danger of oxidation at temperatures up to 550C. Naturally, it is advisable to preheat the graphite for the casting of the first piece and to avoid excessive cooling between successive castings. A permanent regulated temperature ranging from l50 to 400C, depending on they workpiece, its form, its weight and its thickness, may be used;

During the extra-rapid cooling of the liquid steel, the cooling front or liquid-solid interface moves at a speed of up to 30 cm per second for pieces having a thickness of the metal of 6 to l mm, so that the temperature difference between the first and the last solidification points of the workpiece is less than 50C. Under these conditions, the entire workpiece passes through a plastic stage of a certain duration during which either the core is withdrawn from the workpiece or the workpiece from the mold before setting due to cooling shrinkage and the appearance of cracks. To extract a substantially cylindrical workpiece or a core, a taper of l to 2 is sufficient.

The workpieces obtained in accordance with the invention are characterized by the fact that they are cast and hardened at the same time, due to the speed of cooling. This hardening extends through an appreciable part of the thickness of the metal, for example to percent. As a result, excellent mechanical properties are obtained even for common quality raw materials. Specifically, the method in accordance with the present invention offers the advantage of not limiting the characteristics of the steels used as it true for the types of steel which are to be used for stamping or welding. For example, in accordance with the present invention, one can make steel wheels for vehicles with 0.60 percent carbon content, having a tensile strength of 75 to 80 kg/mm while the same wheels could be obtained in accordance with conventional founding methods only from steel having no more than 0.16 percent carbon content and a tensile strength of 40 to 45 kg/mm.

The pneumatic injection molding press in accordance with the present invention includes the following elements which may occupy predetermined positions in relation to a supporting structure:

a. an enclosure capable of holding a liquid metal ladle, connected to a source and to a discharge of compressed gas and provided with a removable lid closing tightly;

b. at least one feed pipe supported tightly by the lid of the enclosure, submerged in the liquid metal bath on one side and emerging from the enclosure on the other side, the head of the feed pipe being connected to the bottom of a mold by means of a joint made of some refractory and plastic material, for example asbestos;

c. a mold supporting slab suspended elastically above the enclosure, comprising a passage for the feed pipe nozzle or for the bottom of the mold as well as mechanism for centering the mold above the feed pipe;

d. a mold to be placed on this supporting slab, consisting of permanent elements, by preference of graphite, assembled in a metal casting box having a feed opening at its bottom and an opening for withdrawal of the cast piece in its upper part;

e. a core, likewise consisting of permanent elements, preferably made of graphite, resting on a metal support;

f. a movable support of the core, moved by jacks for centering the core and the mold as Well immobilizing the workpiece,

closing and opening the mold by inserting and withdrawing the core, and assuring a tight connection between the bottom of the mold and the nozzle of the feed pipe or their separation, by pressure or release of pressure on the mold.

The press further includes mechanisms for the displacement of the enclosure, for example, a carriage, rails and a winch in order to facilitate the refilling of the ladle with metal, mechanisms for moving the mold to withdraw the workpiece and replace the elements used, and control mechanisms for feeding the metal, for moving the core support, and for regulating the cooling circuits.

For a better understanding of the present invention, reference may be had to the accompanying drawings in which:

FIG. 1 is a front view ofa pneumatic injection molding press in accordance with the present invention with the mold removed;

FIG. 2 is a side view in vertical cross section of the press shown in FIG. 1 with the mold present;

FIG. 3 is a plan view of the press shown in FIG. 1 taken at the height of the supporting slab;

FIG. 4 is an enlarged view of the mold shown in FIG. 2, partly in cross section and partly in a front view;

FIG. 5 is an enlarged view of another embodiment of the mold, partly in cross section and partly in a front view.

As shown in FIGS. 1 to 3, the base 10 supporting the entire assembly, except for connected platforms, consists of two longitudinal girders 11 and a cross piece 12.

The base 10 is equipped with rails 13 on which roll the rollers 14 of a carriage 15 consisting of a frame of iron sections and driven by a mechanism 15' shown in broken lines in FIG. 3. The drive mechanism 15' is located at a position distant from the mold 8 and perpendicularly below the mold. The carriage 15 supports a cylindrical container 16, including a metal shell and a layer of suitable insulating material and a bottom consisting of a plate 17 which supports a refractory brick insulating device 18. The container 16 contains a removable casting ladle 19 adapted to receive molten metal 20.

The container 16 has a lid 21, a sealingjoint 22 and a system for bolting the lid to the container. The lid 21 has an opening 23 (FIG. 3) through which extends the feed pipe 24 submerged in the liquid metal 20. The joint between the opening 23 and the pipe 24 is made tight by any suitable means.

The press itself includes a frame structure formed of angle or channel irons 25, fixed to the base 10. This structure carries four columns 26 which are cross braced by an upper beam 27.

A mold supporting slab 28, is suspended elastically on the structure 25 by means of springs 29, which are seated on the frame 25 and on the lugs 30 of the slab, and guided by the columns 26. The slab 28 carries two rails 31 and has a large center opening 32.

A mold-conveying carriage 33 consisting of a frame provided with rollers moves on the rails 31 as well as on the rails 34 and 35 carried, respectively, by the lateral platforms 36 and 37. With this arrangement one mold 8 on the carriage 33 may be in a position to be filled and a second mold (not shown) carried by a second identical carriage (not shown) can be standing by or moved to withdraw the workpiece above either one of the platforms 36 or 37.

The upper crossbeam 27 supports the main jack or cylinder 40 acting on the compensation bar 41 and the rods 42. These rods 42 are guided by rollers 43 mounted on springs (not shown) capable of expansion and fixed on the crossbeam 27. The rods 42 are attached at their lower ends to a brace 44 carrying the core 74 of the mold 8 by means of core support 45. The rods 42 also carry at their lower ends two jacks 46 which, in relation to the brace 44, act on the ring 47 for immobilizing the mold 8 and the work piece 80 and centering the mold. The crossbeam 27 also carries a motor-driven hydraulic pump 48, an oil tank 49 including the distributing valves, and a protective hood 50.

FIG. 4 shows a mold structure 8 in front view and in cross section. The mold 8 includes a metal plate 60 of circular shape housing the various pieces in contact with the liquid metal, namely, the head 61, the bottom 62 and the body 63, all made of graphite. The head 61 is attached by means of the metal flanges 64 and 65 to the bed plate 60 with the screw 66. Between the pipe 24 and the head 61 is inserted an asbestos washer 67 acting as joint. The body 63 includes a detachable collar 68 by means of which it can be serviced and centered, due to the conical shape of its surface in contact with the ring 47. Above the collar 68 rises the ring 47 acted on by the jacks 46 by means of the fingers 69 fixed on the supporting ring 70.

The core support 45 is connected with a metal plate 71 by means of the brackets 72. The plate 71 carries a ring 73 which is conically shaped and which assures locking of the graphite core 74 against the plate. The rings 47 and 73 have shallow recesses in their surfaces which are in contact thereby forming vents, and they assure centering of the core 74 in relation to the mold 8 due to their conical surface of contact.

A tube 75 is mounted on the core support 45 and has attached to it a cap 76 to enable it to be fixed to the graphite bowl 77 by means ofthe pins 78.

An axial bore 79 in the core support 45 enables a liquid to be injected into the support for cooling the bowl 77 which liquid, as a result of the permeability of the graphite, can enter between the cast piece and the core thereby facilitating the withdrawal of the core.

A workpiece 80 is molded in the space between the core consisting of the parts 74 and 77 and the mold proper consisting of the parts 61, 62, 63. A typical workpiece shown in FIG. 4 is a wheel for a heavy duty vehicle having a fixed flange 81, a rim base 82 and a wheel disk 83.

The operation of the press is as follows:

A carriage provided with a ladle 19 filled with liquid metal 20 is placed in feed position under the feed pipe 24 and a carriage 33 carrying a mold 8 is positioned above the pipe 24. The core 74 is then inserted into the mold by means of the jack 40. Centering of the mold is accomplished by means of the rings 73 and 47, the latter serving to fix in position and center the carriage 33. Thus, the pressure of the jack determines the proper position and the closing of the mold. Thereafter, the pressure of the jack, compressing the springs 29, pushes the mold supporting slab 28 until the head 61 is in contact with the pipe 24 via the gasket 67. At this stage, the mold 8 is ready to be filled with the metal 20. To accomplish the filling, the enclosure 16 is put under pressure causing the liquid metal 20 from the ladle 19 to be expelled into the mold 8 through the pipe 24. The metal fills the cavity 80, expelling the air contained therein through the vents formed by the joints of the rings 47 and 73 until the metal solidifies in the vents and closes them. The pneumatic pressure of the enclosure 16 is continued or increased during progressive solidification of the workpiece 80, until the solidification front reaches the head 61. To facilitate controlled cooling and solidification the body 63 made of graphite has a wall of variable thickness, in the example shown, decreasing in thickness from the top towards the bottom.

When the solidification front reaches the head 61, the pneumatic pressure of the enclosure 16 is decreased to atmospheric pressure, while the pressure of the jack 40 is relieved. The

. springs 29 cause the mold supporting slab 28 and the mold 8 to rise again whereby the continuity of the metal is interrupted at right angles with the joint 67 As the pressure of the jack 40 is relieved, the jacks 46 are put under pressure, acting on the ring 47 to hold the cast piece 80 during withdrawal of the core.

The core 74 having been removed, the carriage 33 is moved onto one of the lateral platforms 36 or 37 where the piece 80 is taken out of the mold 8 by means of a gripping and lifting device (not shown) while a second mold-carrying carriage 33 (not shown) is placed in feeding position and the casting cycle is repeated.

When a piece such as the wheel in FIG. 4 is being cast, the

cycle may be accomplished in about 10 seconds. The piece thus obtained displays a surface quality sufficient to make a finishing procedure unnecessary. Only the casting hole after cooling as well as the groove destined to house the other edge of the rim need be machined and holes for attaching the wheel half of the ring. Between the pipe 94 and the head 92 is inserted an asbestos washer 95 as a joint.

The upper part of the mold forming a graphite cover 96 is supported by means of a bearer ring 97 and fingers 98 carrying a plate 99 attached by means of a screw 100. The cover 96 is fixed to the plate 99 by means of pins 101.

A core support 102 comprises in its lower portion a cavity- 103 which is closed off by a joint 104 and cooled by circulation of a fluid, the intake and discharge openings for which are shown at 105. Vents 106 facilitate the escape of the gases contained in the mold and are sealed rapidly by the solidified metal.

A workpiece 107 is molded in the space between the graphite mold elements 91, 93 and 96 which constitute the mold. The workpiece 107 has two projections 108 and 109 which are to be removed by machining.

When casting a piece such as the reel shown in FIG. 5, it IS necessary to dismantle the mold surrounding the piece after molding by the press. Towards such end, the two shells 93 and the piece 107 are withdrawn by a suitable handling device which grips the assembly, and transports it to a molding bench where the two shells 93 are separated to free the piece 107.

It will be obvious to those skilled in the art that many modifications could be made to the present invention without departing from its spirit and scope. The present invention is therefore not deemed limited except as defined in the appended claims.

I claim:

1. A method of castingsteel articles comprising:

a. forcing molten steel by fluid pressure upwardly through the bottom of a graphite mold cavity while venting gas through at least one vent in the top of said cavity, said vent being small in relation to said cavity and being closed by freezing of said steel therein when said cavity is full;

b. increasing said fluid pressure applied to said liquid steel after freezing of said steel in said vent to compact said liquid steel in said cavity;

c. regulating the cooling rate of said steel so as to cause a liquid-solid interface to advance from the region of said vent to said bottom of said mold and to arrive at said bottom of said mold while the steel in said cavity is in a plastic condition;

d. reducing said fluid pressure abruptly when said liquidsolid interface reaches the lowest point in said mold; and e. said reduction in pressure permitting dropping of the liquid steel to separate the cast article when said liquidsolid interface reaches the bottom of said mold.

2. The method in accordance with claim 1 wherein the entire process is conducted within an interval of time such that, when said liquid-solid interface reaches the bottom of said mold, the temperature difference between the steel in the region of said vent and the steel at the bottom of said mold does not exceed approximately 50C.

. 3. The method in accordance with claim 1 wherein said casting is effected within less than l second per kilogram of steel.

4. The method in accordance with claim 2 wherein said article is cast and hardened simultaneously. 

